Position measuring device and method for determining a position

ABSTRACT

A method for position determination in a position measuring device that includes digitizing analog position data from a detector unit within time intervals of an internal clock rate and calculating position data from the digitized data. Determining a period of time Δt between a pulse of the internal clock until an appearance of an external trigger signal and a processing of at least two position data, together with the period of time Δt.

Applicants claim, under 35 U.S.C. §§ 120 and 365, the benefit ofpriority of the filing date of Sep. 15, 2001 of a Patent CooperationTreaty patent application, copy attached, Serial Number PCT/EP01/10682,filed on the aforementioned date, the entire contents of which areincorporated herein by reference, wherein Patent Cooperation Treatypatent application Serial Number PCT/EP01/10682 was not published underPCT Article 21(2) in English.

Applicants claim, under 35 U.S.C. § 119, the benefit of priority of thefiling date of Oct. 31, 2000 of a German patent application, copyattached, Ser. No. 100 54 070.8, filed on the aforementioned date, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a position measuring device, inparticular for employment in connection with a lithographic system. Thepresent invention moreover relates to a method for positiondetermination in a position measuring device.

2. Description of the Related Art

It must be possible by a position measuring device in a lithographicsystem to determine the position of a movable object in such a way thatthe time of the measurement, as well as the position of the movableobject are very precisely fixed. A lithographic system operating inaccordance with the scanner principle moves a wafer table and aphoto-masking table, for example, in relation to each other, and in theprocess exposes only a portion of the area to the photo-mask to berepresented on a wafer. Spatial or chronological errors in positioning,and therefore in position determination, result in overlay errors of thestructures to be represented with respect to structures already presenton the wafer.

In such a lithographic system a central control requests a positionalvalue from a position measuring device at fixed times (for example every50 μs), which then must output the position of the movable object (forexample the mask table) at the time of the position value request as theresponse. Because of the high displacement speeds customary today(approximately 2 m/s for the mask table), an inaccuracy of onenanosecond in the chronological determination of the positional valuethen means an error of two nanometers in the position determination,which approximately corresponds to the demands made on such a positionmeasuring device.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore an object of the present invention to disclose aposition measuring device which can determine the position of a movableobject at a time fixed by an exterior source and thereafter can outputit.

This object is attained by a position measuring device for determining aposition of a movable object that includes a detector unit and an A/Dconverter connected to the detector unit, the A/D converter iscontrolled by an internal clock. A processing unit connected to the A/Dconverter, an extrapolation unit connected to the processing unit and atimer connected to the extrapolation unit and the internal clock, thetimer detects a period of time Δt between a pulse of the internal clockand an external trigger signal.

It is a further object of the present invention to disclose a methodwhich permits the extremely accurate determination of the position of amovable object at the time a position request is made, and subsequentlyto make this available for further processing.

This object is attained by a method for position determination in aposition measuring device that includes digitizing analog position datafrom a detector unit within time intervals of an internal clock rate andcalculating position data from the digitized data. Determining a periodof time Δt between a pulse of said internal clock until an appearance ofan external trigger signal and a processing of at least two positiondata, together with the period of time Δt.

The principle of the present invention is based on determining theposition of the movable object at intervals which are predetermined byan internal clock device of the position measuring device, to determinethe chronological distance of a trigger signal from the last positionmeasurement when a position request (trigger signal) arrives from acontrol unit, and to extrapolate a positional value at the time of thearrival of the trigger signal from at least two previously measuredpositional values. This extrapolated positional value is then output asthe response to the position request.

Further advantages, as well as details of the present invention ensuefrom the following description of a preferred embodiment by thedrawings. Shown are in:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an embodiment of a position measuring devicein accordance with the present invention, and

FIG. 2 shows a diagram showing a possible chronological sequence of theposition determination performed by the position measuring device ofFIG. 1 in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

FIG. 1 shows a position measuring device 1, whose components and theirfunctions will be explained in what follows. Analog signalscorresponding to the physical measuring principle used are generated ina detector unit 2, which can be a scanning head 10 of an optical gratingmeasuring system 12. These can be, for example, signals from aphoto-detector, which receives modulated light from the scanning of anoptical grating measuring system 12 with an incremental graduation 14and reference pulses. As schematically shown in FIG. 1, the opticalgrating measuring system 12 is associated with an object 16, whoseposition is to be determined in accordance with the present invention.Possible embodiments for the optical grating measuring system 12 arewell known in the art. The use of optical measuring principles suggestsitself because of the high resolution required in the above mentionedcase of application (it would also be possible to detect signals from aninterferometer in the detector unit 2), but in principle other measuringmethods are also possible, based on magnetic, inductive or capacitiveeffects, which scan linear, as well as rotary movements. In the case ofapplication described, the detector unit 2 detects the linear movementof a mask table associated with the moving object 16 (see arrowrepresenting linear movement).

The analog signals from the detector unit 2 are conducted to an A/Dconverter 3. The latter digitizes the analog signals respectively attimes t preset by an internal clock 6 of 1 MHz at intervals of 1 μs andpasses the digitized values on to a processing unit 4, in which apositional value is calculated from them. The position measurement atthis fast rate is necessary in order to make possible the counting ofthe increments in an incremental measuring system. It is also possibleto perform corrections, for example of the phase position of theindividual digital signals in the processing unit 4, the resolution ofthe position determination can also be increased by interpolation. Anabsolute position determination can be performed by the evaluation ofreference pulses and the counting of increments of the signals.

The positional values obtained in this way are then passed on to anextrapolation unit 5. Reference is made to FIG. 2 in regard to thefurther course of the position measurement.

If now the position measuring device 1 receives a request (triggersignal 9) for a position measurement from a higher control unit 8, theperiod of time Δt from the last pulse of the internal cycle at tn untilthe arrival of the trigger signal 9 at the time t0 is determined in atimer 7. The timer 7 can be a time-to-digital converter (TDC), forexample, which is set to 0 by the internal clock 6 and internallymeasures a period of time Δt very exactly, which is digitally outputafter the receipt of the trigger signal 9.

This period of time Δt is also sent to the extrapolation unit 5. Anextrapolation for the position at the time t0 is performed there by afunction which is approximated to at least the two position values P1and P2 detected at the times tn and tn−1. This has advantages: with anobject at rest, a linear extrapolation with two position values resultsin the mean of the position, and therefore a lowering of the noise. Foran object moved at a constant speed the extrapolation is still veryaccurate. For obtaining advantages in regard to noise reduction it isnecessary to extrapolate a straight line by three support points. Foraccelerated systems it is necessary to fall back on higher orderfunctions (for example polynomials) in order to still obtain accurateextrapolations. However, if the maximally occurring accelerations areknown, a limitation to linear extrapolation is possible if the erroroccurring because of this remains sufficiently small. For this purpose,the actual position course P(t) of an object being accelerated in onedirection is shown in FIG. 2, together with the two positional values P1and P2 detected at the times tn and tn−1. ΔP shows the error in aposition calculation with linear extrapolation of a straight line Gthrough P1 and P2. In the described case of application, accelerationsof the mask table up to 10g must be expected, therefore, with aninternal clock 6 of 1 MHz, a maximum extrapolation error ΔP of 0.05 nmresults for Δt =1 μs. Based on the above extrapolation process, it isapparent that more position values are determined at previous time tn,tn−1 . . . and are employed than mathematically necessary.

The positional value determined in this way therefore is anapproximation of the required positional value of the moved object atthe time t0. It is transmitted from the position measuring device 1 tothe higher control unit.

For optimally utilizing the advantages of noise suppression by takingthe mean, on the one hand, and the exact determination of the positionat the time t0, on the other hand, it would also be conceivable to matchthe extrapolation flexibly to the respective situation. Thus, a linearextrapolation with many points can take place in case of a slightlyaccelerated movable object, and a large noise suppression can beachieved in this way, but with greatly and not constantly acceleratedobjects a fall-back to an extrapolation of higher order can take placefor keeping the deviation ΔP low.

It is a further advantage of this position measuring device that it ispossible to take signal running times, which otherwise might falsify themeasured result, into consideration. For example, light requires adefinite time from a scale to the detector unit 2, the electronicfollow-up devices in the detector unit 2, in the A/D converter 3 and inthe processing unit 4 further contribute to delays in the measuringsignal. These effects can be taken into consideration by an offset ofthe period of time Δt determined in the timer 7.

The present invention may be embodied in other forms than thosespecifically disclosed herein without departing from its spirit oressential characteristics. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive, andthe scope of the invention is commensurate with the appended claimsrather than the foregoing description.

1. A position measuring device for determining a position of a movableobject, comprising: a detector unit; an A/D converter connected to saiddetector unit, said A/D converter is controlled by an internal clock; aprocessing unit connected to said A/D converter; an extrapolation unitconnected to said processing unit; and a timer connected to saidextrapolation unit and said internal clock, said timer detects a periodof time Δt between a pulse of said internal clock and an externaltrigger signal.
 2. The position measuring device in accordance withclaim 1, wherein said extrapolation unit extrapolates a position from atleast two previously measured position data and said period of time Δt.3. The position measuring device in accordance with claim 1, whereinsaid detector unit comprises a scanning head of an optical gratingmeasuring system.
 4. A method for position determination in a positionmeasuring device, comprising: digitizing analog position data from adetector unit within time intervals of an internal clock rate;calculating position data from said digitized data; determining a periodof time Δt between a pulse of said internal clock until an appearance ofan external trigger signal, wherein said period of time Δt is measuredby a timer; resetting said timer by said internal clock; outputting saidperiod of time Δt elapsed since a last resetting of said timer and areceipt of said trigger signal by an extrapolation unit that performssaid processing; and processing of at least two position data, togetherwith said period of time Δt.
 5. The method in accordance with claim 4,further comprising: extrapolating a positional value at a time ofarrival of said external trigger signal by using a polynomial function.6. The method in accordance with claim 5, further comprising:determining said polynomial function, wherein more position values aredetermined at previous times tn, tn−1, . . . and are employed thanmathematically necessary; and obtaining a means for noise suppression.7. A method for position determination in a position measuring device,comprising: digitizing analog position data from a detector unit withintime intervals of an internal clock rate; calculating position data fromsaid digitized data; determining a period of time Δt between a pulse ofsaid internal clock until an appearance of an external trigger signal;processing of at least two position data, together with said period oftime Δt; and extrapolating a positional value at a time of arrival ofsaid external trigger signal by using a polynomial function.
 8. Themethod in accordance with claim 7, further comprising: determining saidpolynomial function, wherein more position values are determined atprevious times tn, tn−1, . . . and are employed than mathematicallynecessary; and obtaining a mean for noise suppression.